The invention generally relates to power conductor rails used for electrically powered rail vehicles and more specifically to power conductor rail expansion joints.
Electrically powered rail vehicles have long been used for mass transit systems. Electric rail systems typically employ a three rail configuration, the rail system having two running rails to support the vehicle and a third rail to conduct the necessary electrical power. The vehicle has a contact shoe which extends out from the vehicle and makes electrical contact with the third rail.
In the late 1960s, Metro Transit Authorities and supporting manufacturing companies began experimenting with different rail structures for third rails in an effort to reduce electrical resistance. Various conductor rail constructions evolved from these experimental efforts. As rail structures began to take new shapes, research regarding the methods of connecting rails sections quickly followed.
A particular problem associated with power conductor rail systems is the need for power continuity along the rail length. Conductor rails are typically comprised of a series of linked rail sections which are connected linearly in the field during installation by mechanical splice joints or welded joints to establish a long continuous rail assembly. Thermal expansion elongation of the rail must be absorbed by the length of the rail system loop. Conventionally, mechanical expansion joints made of steel are incorporated into the rail length at joining rail sections. The adjoining rail sections would have to be connected by a series of power cables in an attempt to keep power transfer continuous. Even with this type of construction, power continuity can never realistically be achieved because the power transfer cables are of a different resistance than that of the rail. Additionally, power transfer cables tend to establish electrical "hot spots" under power flow. Consistent electrical resistance throughout the full length of the power rail is necessary for an efficient power rail system but impossible to obtain using conventional constructions.
In an effort to improve power transfer, various expansion joint structures for conductor rails have been designed. An example is that taught by U.S. Pat. No. 3,790,725 to Charamel et al. issued Feb. 5, 1974. Another example in U.S. Pat. No. 3,689,713 to Shkredka issued Sep. 5, 1972. These expansion joint constructions, however, are complex and extremely expensive to fabricate and install making such structures nearly cost prohibitive under most circumstances.